res_pjsip_refer: Prevent sending duplicate headers.

res_pjsip_refer: Prevent sending duplicate headers.

res_pjsip_refer will attempt to add Referred-By or Replaces headers to
outbound INVITEs at times. If the INVITE gets challenged for
authentication, then we will resend the INVITE. Prior to this patch, the
Referred-By or Replaces header would be re-added to the outbound INVITE,
resulting in duplicated headers.

ASTERISK-25204 #close
Reported by Mark Michelson

Change-Id: I59fb5c08b4d253c0dba9ee3d3950b5025358222d